Bug 463808

Summary:	Quickly pressing alt+tab in succession from a window that is capturing the cursor won't keep mouse focus
Product:	[Plasma] kwin	Reporter:	MScattolin <matheus.scattolinanselmo>
Component:	wayland-generic	Assignee:	KWin default assignee <kwin-bugs-null>
Status:	REPORTED ---
Severity:	normal
Priority:	NOR
Version First Reported In:	5.26.5
Target Milestone:	---
Platform:	Arch Linux
OS:	Linux
Latest Commit:		Version Fixed/Implemented In:
Sentry Crash Report:
Attachments:	alt+tabbing from a game (TF2) and back

Description MScattolin 2023-01-03 23:11:56 UTC

SUMMARY
This does not happen in the X11 session, only wayland is affected here.
If the current focused window is capturing the pointer, pressing alt+tab twice (apparently under 1s), effectively switching to another window and quickly going back,  the mouse focus is completely lost, as in you can't interact with the window using the mouse pointer anymore. 
This is particularly obvious in full screen games, as you can't use the mouse within the game window after alt+tabbing twice, yet the cursor is visible on the screen, it's simply not relayed to the window anymore.
The workaround is to manually minimize the window or pressing alt+tab again, then waiting a second, then alt tabbing back. It kinda feels like there's a timer component to it

Keyboard focus in this scenario works as expected regardless

STEPS TO REPRODUCE
1. Open fullscreen game: native/wine, it doesn't matter
2. Press alt+tab twice quickly
3. Cursor is visible on top of the window, but you can't interact with it anymore

Comment 1 MScattolin 2023-01-28 18:43:03 UTC

Created attachment 155740 [details]
alt+tabbing from a game (TF2) and back

Here's a recording for illustration purposes, I'm alt+tabbing out of a game and back, then doing it again after a short delay, if you do it too fast, the game won't grab the cursor anymore. Then I repeated but after waiting a short while, and it worked.
I originally stated that it happens in any game, but it's not true, I've found exceptions. However, I have no idea why some do, some don't, but it happens every time with ones who do. Being fullscreen and changing the cursor or just grabbing it seems to be related, but as it is now, the pattern is somewhat unclear
Also, I should note that it only happens when using the alt+tab shortcut, manually clicking on the taskbar icon works as expected every time.

I'm on plasma 5.26.5, but tested with 5.27 beta to same results, X11 session remains unaffected. I'm also using an AMD GPU on mesa drivers.